Conventionally, there is known an illumination apparatus provided with a sensor for detecting the presence or absence of a human body and designed to turn on or off a lamp depending on the presence or absence of a human body, one example of which is disclosed in Japanese Patent Laid-open Publication No. 2001-325810. In case where a passive sensor, e.g., a heat ray sensor, for detecting infrared rays emitted from a human body is used as the human sensor, it is necessary to expose the sensor toward a detection direction. This poses a problem in that the sensor imposes a limitation on the structure of the illumination apparatus and adversely affects the outward appearance thereof. In order to enable the sensor to detect a human body even when installed within the illumination apparatus, it may be conceivable to use a so-called active sensor capable of transmitting energy waves such as electromagnetic waves or the like and receiving the energy waves reflected from a target object to detect the presence or absence of the target object depending on the existence and nonexistence of a Doppler frequency, namely the difference between the frequency of the transmitted waves and the frequency of the received waves.
One conventional illumination apparatus provided with the afore-mentioned active sensor will now be described with reference to FIG. 6. As illustrated in FIG. 6, the conventional illumination apparatus includes a housing body 100 of elongated box shape with one side opened, a pair of sockets (not shown) arranged in the longitudinal opposite end portions of the housing body 100, a straight tube type discharge lamp La held by the sockets, a sensor unit 101 installed within the housing body 100 at the opposite side to the open side of the housing body 100 with the discharge lamp La interposed therebetween, a reflection plate 102 of elongated plate shape arranged on the inner surface of the housing body 100 for reflecting the electromagnetic waves transmitted from the sensor unit 101 and irradiating the same toward the outside of the housing body 100 through the open side, and a control unit 103 for performing a control operation to turn on the discharge lamp La in response to the detection signal supplied from the sensor unit 101. In the conventional illumination apparatus, the sensor unit 101 is arranged substantially at the longitudinal center of the discharge lamp La and on the opposite site to the light irradiation direction of the discharge lamp La in order to meet the demand for smaller transverse dimension and improved outward appearance of the housing body 100 and the demand for the sensor unit 101 to have a uniform detection range about the housing body 100.
In the conventional illumination apparatus, however, the waves transmitted from the sensor unit 101 are reflected by the electrons present inside the discharge lamp La because the discharge lamp La exists within the detection range of the sensor unit 101. Since the electrons present inside the discharge lamp La are moving during the turned-on period of the discharge lamp La, a Doppler effect is generated as in the case where electromagnetic waves are reflected from an object to be detected. During the turned-on period of the discharge lamp La, therefore, the sensor unit 101 continues to work as if it has detected a target object. This makes it impossible to accurately detect the presence or absence of a target object.